The nuclear pore complex (NPC) is one of the most important molecular machines in eukaryotes because it gates the porous conduits between the cytoplasm and nucleoplasm of cells and controls all nucleo-cytoplasmic traffic and communication. Its most important architectural feature is a poorly understood semi-permeable diffusion barrier in its center that maintains a tight seal against cytoplasmic proteins as small as 4 nanometers in size, but opens to allow facilitated transport of particles of all shapes and sizes up to 40 nanometers in size. This flexible barrier is composed of a family of filamentous proteins named FG nucleoporins (FG nups) that feature large unfolded domains in their native functional state, which are decorated with multiple phenylalanine glycine motifs (FG domains). The specific aims of our proposed research are to 1) characterize the dynamic structure and intra-molecular interactions of FG domains representing two different types of FG nup filaments that are anchored at three different locales of the NPC, and 2) test the hypothesis that inter-molecular associations between FG domains of nups create a filamentous meshwork structure at the NPC center, which establishes the size-selective barrier to the passive diffusion of proteins. The proposed experiments will combine biochemical, biophysical, cell biological, structural (Nuclear Magnetic Resonance), and molecular modeling techniques to gain insight into the dynamic behavior, structure and function of the FG nups. We will also gain fundamental knowledge on the dynamic behavior and structure of disordered domains of proteins in general. We are studying the three-dimensional structure of the cellular proteins that function as gatekeepers of our genetic material in the nucleus. Their proper architecture and function is vital to human health because they control the flow of information to and from our genes. When they fail to function normally, these proteins can trigger the onset of cancer. We wish to understand how their structure enables them to function as gatekeepers of the nucleus.